As is generally known, in many cases, a plurality of electronic control units (ECU) mounted in a vehicle configure a communication system in which pieces of information (vehicle information) possessed by the electronic control units are communicable to each other by allowing the respective electronic control units to make a network connection to each other. A controller area network (CAN) is as one of such vehicle network systems.
In the CAN, arbitration related to message transmission is performed based on a message ID, which is an ID particular to a message, when message transmissions from ECUs coincide with each other. Thereafter, only the highest-priority message among messages whose transmissions coincide with each other is permitted to be transmitted, and the other messages are kept waiting to be transmitted. In the CAN, the number of cases in which message transmissions coincide with each other becomes larger in proportion to an increase in messages that flow through the bus. Therefore, the possibility that a delay will occur in message transmission rises. Therefore, techniques for reducing a delay caused when messages are transmitted have been heretofore proposed, and its one example is described in Patent Document 1.
A data transfer unit described in Patent Document 1 is a device that transfers messages between two communication buses. The data transfer unit includes a first frame control module that transmits/receives messages to/from a first communication bus, a second frame control module that transmits/receives messages to/from a second communication bus, and a microcomputer that is connected to these two frame control modules and is capable of transferring messages therebetween. The microcomputer is provided with a communication bus load monitoring device, which measures the bus load of the first communication bus or that of the second communication bus. The data transfer unit additionally includes a first receive-only module, which transfers a message received from the first communication bus to the second frame control module, and a second receive-only module, which transfers a message received from the second communication bus to the first frame control module. Each receive-only module includes a reception selecting register. The microcomputer sets, in the reception selecting register, a shielding rate, according to which the transfer of a message to a frame control module that is a transfer destination is restrained. When the bus load of the second communication bus becomes 50% or more, the microcomputer sets the shielding rate of the reception selecting register of the first receive-only module at a low shielding rate for a message having high priority and at a high shielding rate for a message having low priority, so that the delay of the message having high priority is reduced.